1. Field of the Invention
The present invention relates to a magnetic tape, in particular, to a magnetic tape used as an external recording medium for recording data, which includes a magnetic layer composed of evaporated magnetic films.
2. Description of the Related Art
Recently, along with the diffusion of minicomputers, personal computers or office computers such as work stations, a magnetic tape for recording computer data as an external recording medium (a so-called backup tape) has been actively studied.
For practical use of such a magnetic tape for data storage, the increase in storage capacity is highly demanded in relation with, in particular, the miniaturization of computers and the increase in information processability. In addition, with the expansion of the environment where computers are used, the magnetic tape is now used under a broad range of environmental conditions (in particular, under the conditions where a humidity and a temperature greatly fluctuate).
Therefore, the magnetic tape is more and more strongly required to have high tape running durability and the capability of stably recording data after repeated use at high speed, reliability in view of performance such as readout performance, reliability in stable reproduction even after stock for a long period of time and the like, even in such an expanding environment.
Conventionally, as a magnetic tape used for data storage, a coating-type magnetic tape is now widely used. This type of magnetic tape is fabricated by applying and drying a magnetic coating material, which is obtained by dispersing a powered magnetic material in an organic binder, onto a nonmagnetic support. In order to ensure long-term reliability of the coating-type magnetic tape, various conditions are defined for the magnetic tape (see JP 11-96545 A).
On the other hand, with an increasing demand for high-density magnetic recording mentioned above, a metal magnetic thin-film type magnetic tape, a so-called metal-evaporated tape, has been proposed and attracted attention. In this type of magnetic tape, a metal magnetic material is directly deposited onto a nonmagnetic support by plating or vacuum thin-film formation means.
Two types of recording/reproduction system for a magnetic tape used for data storage have been put into practical use, namely a helical scan system and a linear system. For the data storage applications where a volume is not strictly limited, the linear system is now practically used. Moreover, in the commercial market, products such as a digital linear tape (DLT) or a linear tape-open (LTO) become predominant.
The linear system is for providing tracks in a width direction of a magnetic tape so as to bidirectionally recording and reproducing data in a longitudinal direction. This system is advantageous in facility of allowing a magnetic tape to run at high speed and in capability of improving a transfer rate of recording and reproduction by arranging a plurality of magnetic heads in parallel.
As a magnetic tape used for data storage in the linear system, not an obliquely evaporated magnetic tape but only the coating-type magnetic tape has been used. The reason for this is as follows. Since a magnetic tape bidirectionally runs in the linear system, characteristics such as the optimum recording current, a phase characteristic, a C/N ratio and an output characteristic greatly differ between a forward longitudinal direction and a reverse longitudinal direction of the tape in recording and reproduction.
As a method for solving such a problem, an obliquely evaporated tape including a magnetic layer composed of two obliquely evaporated films, each having a different growth direction of oblique evaporation, has been proposed (see JP 4-353622 A and JP 10-198940 A). In this context, the obliquely evaporated magnetic tape starts attracting lots of attention again as a magnetic tape medium for data storage used in the linear system.
As mentioned above, along with rapid diffusion of personal computers and the like, the magnetic tape for recording computer data is now frequently used under the environments where a temperature and a humidity greatly fluctuate. Therefore, in the case where the obliquely evaporated magnetic tape is used, it is desired that the recording and reproducing operation do not produce any errors and have high reliability in terms of stable data input/output even when the magnetic tape is used under severe environmental conditions as mentioned above.
In particular, since a total thickness tends to be reduced so as to achieve large recording capacity, a tape itself is required to have higher rigidity and wear resistance than ever before. In addition, the magnetic tape is required to have a lower degree of deformation with respect to a change in temperature and humidity and therefore have excellent form (dimensional) stability.
Moreover, in order to achieve a higher recording density, a system enabling the data recording and reproduction in a narrower track width than a conventional one is now under development. Therefore, a metal-evaporated tape having high form stability, which is sufficiently compatible with such a magnetic recording/reproduction system, is required.
As a result of the studies made by the inventor of the present invention, however, it is found that the metal-evaporated tape is not still satisfactory in terms of form stability in the case where it is used as a conventional magnetic tape for computer data recording. For example, generally, a magnetic tape is likely to be extended in a longitudinal direction in its fabrication step while it is likely to gradually shrink during stock after the fabrication. Even in the case where the metal-evaporated tape is used for conventional computer data recording, a shrinkage ratio is relatively large. Therefore, if recording/reproduction is performed on/from this tape, it is found that tracking accuracy is lowered. Because of the thus lowered tracking accuracy, a reproduction output is likely to be lowered.
In the case where a fabricated metal-evaporated tape is stocked at a high humidity, it is found out that the tape absorbs moisture to be easily deformed. As a result, a reproduction output is also likely to be lowered as described above.